Question: If there is a bubble in the conformal coating that covers two traces, but both traces are covered with soldermask is this rejectable? Also, how do you avoid bubbles in the coating?
Answer: There is much information on conformal coating on the web and a simple search in Google will provide you with some additional articles to review.
In the process of reading your question I would consider the attached statement from the J-STD-001 document. Since the bubble is on top of the soldermask coating and if the bubble was not there would the noncommon conductors be exposed? Would the adjacent component leads be exposed? If the answer to both of these questions is NO, then I would think they do not meet the defect criteria and should be considered process indicators.
The following has been extrapolated from the J-STD-001 Rev F:
10.3.5 Bubbles and Voids Cured coating should be free of bubbles and voids.
When present, bubbles and voids shall not [D1D2D3]:
- Bridge noncommon leads or conductors.
- Expose one or more conductors.
When bubbles and voids are present and do not meet the defect criteria, they shall [P1P2P3] be considered as process indicators.
How can bubbles be eliminated from the conformal coating? I would consider this a workmanship issue and focus of the preparation of the material, such as stirring or blending of the material being too aggressive, which would created bubbles within the liquid mass of the conformal coating. If the application is sprayed, again I would look at the amount sprayed and the physical spraying process.
At times one of the ways to eliminate bubbles or degassing viscous liquid is to evacuate the material in a vacuum system prior to using and applying the material.
Check the following website:
“A method and apparatus for degassing viscous liquids and removing gas bubbles suspended therein is provided, the method comprising allowing a liquid to flow into a gas tight vessel which is under reduced pressure, the liquid first passing an apertured partition which causes many more small bubbles to form in the liquid before it passes into the partially or fully evacuated vessel. The liquid therein is subject to ultrasonic vibrations which coagulates the numerous small bubbles with each other and with other bubbles causing them to rise to the surface of the liquid where they are drawn off by a vacuum pump.”